Methane seepage in the northwestern part of the German North Sea

We studied seafloor characteristics, water column anomalies, and sediment methane geochemistry in the German sector of the central North Sea during a research cruise with the German research vessel Heincke in summer 2019. An extensive hydroacoustic mapping campaign revealed the presence and distribution of flares in the water column, indicative for gas bubble releases as well as for geophysical subsurface indications of elevated gas concentrations. We analyzed the spatial distances of detected flares to subsurface salt diapir locations, seismically identified gas accumulations, and abandoned well sites. Continuous and discrete measurements of dissolved methane concentrations in the water column support the identification of seepage from the seafloor. Our data demonstrate that dissolved methane concentrations in the upper water column were not enriched above the studied well sites. At one area, characterized by the presence of shallow gas pockets, we observed methane concentrations ten times enriched compared to background values close to the seafloor. Our results indicate an active natural seep system in the northwestern part of the German North Sea, which is related to updoming salt structures rather than leaking wells, and further underlines that natural seeps may challenge the identification of potentially leaking wells. Due to the shallow water depths of 30 to 50 m in the study area, at least part of the released methane is probably contributing to the atmospheric inventory. This conclusion is based upon our observations of flares reaching close to the sea surface and a slight oversaturation of surface waters in the flare-rich area.